1. Field of the Invention
This invention relates to electrical devices comprising conductive polymer compositions, to methods of making such devices, and to circuits comprising such devices.
2. Introduction to the Invention
Electrical devices comprising conductive polymer compositions are well known. Such devices comprise an element composed of a conductive polymer. The element is physically and electrically connected to at least one electrode suitable for attachment to a source of electrical power. The factors determining the type of electrode used include the specific application, the configuration of the device, the surface to which the device is attached, the resistance of the device, and the nature of the conductive polymer. Among those types of electrodes that have been used are solid and stranded wires, metal foils, perforated and expanded metal sheets, porous electrodes, and conductive inks and paints. When the conductive polymer element is in the form of a sheet or a laminar element, metal foil electrodes that are directly attached to the surface of the conductive polymer, sandwiching the element, are particularly preferred. Examples of such devices are found in U.S. Pat. Nos. 4,426,633 (Taylor), 4,689,475 (Matthiesen), 4,800,253 (Kleiner et al), 4,857,880 (Au et al), 4,907,340 (Fang et al), 4,924,074 (Fang et al), 5,831,510 (Zhang et al), 5,852,397 (Chan et al), 5,864,281 (Zhang et al), and 5,874,885 (Chandler et al), the disclosures of which are incorporated herein by reference.
Metal foils having microrough surfaces can give excellent results when used as electrodes in contact with conductive polymers. U.S. Pat. No. 4,689,475 discloses the use of metal foils that have surface irregularities, e.g. nodules, which protrude from the surface by 0.1 to 100 xcexcm and have at least one dimension parallel to the surface which is at most 100 xcexcm. U.S. Pat. No. 4,800,253 discloses the use of metal foils with a microrough surface which comprises macronodules which themselves comprise micronodules. U.S. Pat. No. 5,874,885 discloses the use of a metal electrode made of more than one type of metal with particular surface characteristics. Other documents which disclose the use of metal foils having rough surfaces, but which do not disclose the characteristics of the foils, are Japanese Patent Kokai No. 62-113402 (Murata, 1987), Japanese Patent Kokoku H4-18681 (Idemitsu Kosan, 1992), and German Patent Application No. 3707494A (Nippon Mektron Ltd., 1988). U.S. Pat. No. 5,880,668 discloses the use of a modified polyolefin with a carboxylic acid derivative grafted onto the polymer in combination with certain foils. The disclosure of each of these documents is incorporated herein by reference.
Desired properties of electrode materials for conductive polymer devices include: a low contact resistance to the polymer; a strong bond which will survive extended and repetitive electrical and/or mechanical stresses and adverse environmental conditions such as extreme temperatures, temperature cycling, heat and humidity; compatibility with conventional fabrication techniques; and low cost.
We have found that improved electroding for conductive polymers can be accomplished by using foil having a combination of surface features making up the surface roughness which is in a range lower than that previously used. Until now, the primary mechanism proposed for forming a good bond between conductive polymers and metal foils has been mechanical interlocking achieved by using a rough surface on the metal foil, wherein the surface of the foil is imbedded into the conductive polymer by heating the polymer above its melting point during the electroding process. The resulting devices can have low contact resistance and good electrical performance. However, we have found that excellent performance can be achieved by fabricating devices with metal foils that have a surface roughness parameter described by the product of two characteristic measurements of surface properties. Devices made by the present invention demonstrate low electrical resistance indicating low contact resistance at the electrode-polymer interface, resistance stability following thermal cycling, and improved resistance stability during and following prolonged and repeated electrical stress.
Ra is a measure of one aspect of surface roughness known as xe2x80x9ccenter line average roughness,xe2x80x9d which relates to an average value of the height of protrusions from a surface and is further described below. The measurement of the average value of height of protrusions, however, does not give any information about the density, distribution, or nature of the protrusions (e.g. spiked, rounded, etc.). A measurement of the reflection density RD of the surface (described below) gives a value which relates to the amount of light reflected from a surface using fixed incident light parameters, and thus gives a measure of the amount of structure on the surface on a size scale comparable to the wavelength of the light (i.e., visible light, around 600 nm). A shiny smooth surface will give a low reflection density, as most of the light will be reflected. The combination of Ra and RD can be used to describe the surface of a foil, and it is especially useful to multiply Ra by RD to describe the surface of the foil.
Foils that have lower surface roughness characteristics than those previously used can be less expensive than those with higher surface roughness. In addition, lamination of viscous or highly filled conductive polymer compositions using melt processing can be facilitated by the use of lower structure foils since it is easier to imbed features of smaller average height into the viscous compositions. For example, a faster line speed can be allowed since less time is required for the polymer to flow around and fill in a structured foil surface. In a foil which has a surface which is relatively rough, it is possible that the conductive polymer composition will not fill in completely around the features of the foil surface, resulting in trapped air pockets which disrupt the electrical continuity and provide points of failure at the interface, especially under electrical stress or environmental aging. The use of lower roughness foil with smaller protrusions can enable the effective lamination of conductive polymers at much lower temperatures, which is advantageous for some applications since it is known that some conductive polymer properties can be sensitive to thermal history. We have found that foils with submicron protrusions that are present with sufficient density can make excellent electrical and mechanical bonds with these polymers.
Additionally, an adhesion promoting layer such as a coupling agent can be used between the foil and the conductive element. The use of adhesion promoting layers in combination with foils having certain roughness characteristics is described in copending commonly assigned application Ser. No. 09/606,821 (Becker et al.), filed contemporaneously with this application, the disclosure of which is incorporated herein by reference.
In a first aspect, this invention provides an electrical device comprising
(A) an element which
(1) has first and second surfaces and
(2) comprises a conductive polymer composition, and
(B) a first metal foil electrode which
(1) comprises
(a) a first surface having (i) a center line average roughness Ra xcexcm, and, (ii) a reflection density RD, the product Ra times RD being 0.5 to 1.6 xcexcm, and
(b) a second surface, and
(2) is positioned so that the first surface of the electrode is in contact with the conductive polymer element.
In a second aspect, this invention provides an electrical device comprising
(A) an element comprising a conductive polymer composition and
(B) a first metal foil electrode which
(1) is produced by
(a) providing a base metal foil having a first surface having a center line average roughness Ra of at most 0.45 xcexcm, and
(b) depositing material to provide protrusions onto the first surface of the base metal foil,
(2) comprises
(a) a first surface having (i) a center line average roughness Ra xcexcm and (ii) a reflection density RD, the product Ra times RD being at least 0.14 xcexcm, and
(b) a second surface, and
(3) is positioned so that the first surface of the electrode is in contact with the conductive polymer element.
In a third aspect, this invention provides an electrical device comprising:
(A) an element comprising a conductive polymer composition,
(B) a first metal foil electrode which comprises
(1) a first surface which is attached to the conductive polymer element and has
(a) a center line average roughness Ra xcexcm, and
(b) a reflection density RD, the product Ra times RD being at least 0.14 xcexcm, and
(2) a second surface, and
(C) a crosslinking agent positioned between the first electrode and the conductive polymer element.
In a fourth aspect, this invention provides a process for making an electrical device, said process comprising:
(A) providing an element comprising a conductive polymer composition,
(B) providing a first metal electrode having
(1) a first surface having a center line average roughness Ra and a reflection density RD such that the product Ra times RD is at least 0.14 xcexcm, and
(2) a second surface,
(C) positioning at least one crosslinking agent between the conductive polymer and the first surface of the first metal electrode, and
(D) securing the first surface of the metal electrode to the conductive polymer element with the crosslinking agent therebetween.
In a fifth aspect, this invention provides an electrical device, the device comprising
(A) an element comprising a conductive polymer composition and
(B) in contact with the element, a metal electrode comprising
(1) a base metal foil and
(2) first and second surfaces, said first surface comprising
(a) protrusions having a maximum height of 1 xcexcm and
(b) a reflection density RD of at least 0.6.
In a sixth aspect, this invention provides an electrical device, the device comprising
(A) an element comprising a conductive polymer composition and
(B) a metal electrode, the metal electrode comprising
(1) a base metal foil,
(2) a first surface which
(a) comprises dendritic metal structures, and
(b) is in contact with the element, and
(3) a second surface.
In a seventh aspect, this invention provides an electrical circuit which comprises
(1) a source of electrical power;
(2) a load; and
(3) an electrical device, e.g. a circuit protection device, of the first, second, third, fifth or sixth aspect of the invention electrically connecting the source and the load.